1. Field of the Invention
The present invention relates to a sliding element for seals used such as for mechanical seals and to their process of manufacturing. More particularly, the invention relates to a sliding element for seals that are excellent in lubrication property and can prevent sliding face from surface damage like wear or Blister caused such as by an adhesion or a seizure when sealing the fluid and to their process of manufacturing.
2. Description of the Related Art
Generally, carbon sliding elements are widely used as seal rings such as mechanical seals to seal the fluid. These carbon sliding elements have, in addition to solid lubrication property, moderate uneven portions on sliding surface. Accordingly, fluid film of sealed fluid can be maintained between seal sliding face of carbon sliding elements and that of mating sliding elements. Thus the carbon sliding elements show excellent lubrication property.
However, when the mechanical seals are used under a heavy load condition, though the carbon sliding elements show excellent solid lubrication property, seal sliding faces become in boundary lubrication condition and partially, by solid contact of seal sliding faces, adhesion effect may occur. Consequently, friction coefficient may be notably high and the carbon sliding elements may be short lived due to the wear. Also the sliding faces may be mirror surfaced and as Blister effect, surface blister may occur and consequently, seal function may deteriorate and sealed fluid may leak in cases.
In order to solve these problems, instead of the carbon sliding element, as a material having a high degree of hardness and strength, a hard material represented by silicon carbide may be used. And also as a mating sliding material, a hard material represented by silicon carbide with scattered pores inside may be used. This combination of mechanical seals is occasionally used. However, these mechanical seals have below mentioned problems.
First, although the silicon carbide has a high degree of hardness and is excellent in the wear resistance, it is poor in solid lubrication property. Accordingly, when the combination of silicon carbide elements are used under a heavy load condition, the sliding faces readily become mirror surfaced and while deactivation of sliding, when lubrication film between the sliding faces run out, mirror surfaced seal sliding faces may be secured to each other. This extraordinarily increases a friction coefficient of the initial running of sliding and consequently, seizure may occur in cases.
Secondly, due to the pores scattered inside the matrix of silicon carbide, the incoming sealed fluid into the sliding face may cause liquid storing effect or hydrodynamic effect during sliding, and this may reduce the anomaly or seizure of the sliding torque. However, at any cause, even for a short time, when sealed fluid is not located to a neighborhood of seal sliding face, or when relatively movable sliding faces is not under the fluid lubrication condition at the time of initial running or deactivation of sliding, it will be under the solid contact condition between the sliding faces and consequently be the same condition with the above first problem.
When under such solid contact condition, there are cases in which extreme increase of sliding heat generation occur and secondary seal portion of rubber packing or the like may be damaged by the heat, or materials dissolved in the sealed fluid may be precipitated and accumulated between the sliding faces. Consequently seal function may not stably be maintained.
On the other hand, silicon carbide sliding material with scattered pores and carbon material are combined to use as mechanical seals in cases. In this case, relative to the combination of silicon carbide sliding element, due to the solid lubrication property of carbon sliding material, the abovementioned inconvenience is modified. However, in this case, a hard material having isolated pores and concaves has a sliding motion with relatively soft carbon material. Due to this sliding motion, the hard material scratches the carbon material and this may lead to the increase of torque, further, to the wear of carbon sliding element. Also pores (concaves) on silicon carbide sliding face may be clogged by carbon wearing powder and pores on the sliding face may not maintain the stabled fluid lubrication effect in cases.